Is it possible to test only the father and the child.
However, if the mother takes part in the test, both the inspection accuracy and speed will improve.
Since a child is born from his/her mother, the child's genes and the mother's genes always match.
Without the presence of the mother, even if the half of child's genes match the alleged father's genes, it is difficult for us to judge whether that is accidental or due to a biological relationship.

Our company is a testing company established in the United States. We are offering this service to Japanese speakers that native language is Japanese. In Nevada, you can do the test without permission.

Using blood has the same accuracy level as saliva.
Blood sampling is painful. However, in the case of saliva, you can just collect with a swab from inside the mouth. There is no pain, and the process is very easy.
Furthermore, if you let the swab dry, it can be stored for months at room temperature.

It is possible. In this case, hair, body fluids and blood which are attached to clothing can be used as a special sample. In some cases it may be difficult depending on the condition of the sample.
If you can collect a sample from the man's parents or siblings, the genes of the deceased can be reconstructed, and we can perform the test.